Vehicle having automatic transmission may include auxiliary devices which should be controlled at the time of shifting or corresponding to shift positions, e.g. direct clutch lock-up device, ignition advance device, exhaust processing device and the like. In this case, the automatic transmission must include a shift position detector. Conventionally, such a shift position detector is known, which includes a hydraulic pressure switch operated by detecting working hydraulic pressure which is delivered to friction elements for selecting each shift position of the automatic transmission, e.g. rear clutch, front clutch, band brakes and the like, wherein shifting is detected by ON or OFF of the pressure switch. However, the working hydraulic pressure to actuate the friction elements is not applied at the same instant with the actuation of the shift valve, and is applied with some time lag from the actuation of the shift valve due to delayed response of hydraulic system. Thus, conventional shift detector can not detect the shifting at the actual instant thereof when the shift valve is actuated, and accompanies some actuation delay. Also such a shift detector requires a design modification to add a hydraulic circuit for operating the pressure switch to a hydraulic system of shift control apparatus of automatic transmission. Besides, the pressure switch itself is relatively expensive. Consequently, manufacturing cost of such a shift detector inevitably increases.